1. Field of the Invention
The present invention relates to impedance matching, and more particularly to a method and apparatus for performing impedance transformations between dissimilar circuit input and output load impedances over broad bandwidths. The invention further relates to employing parallel distributed amplifiers in an amplifier structure to obtain large impedance matching ratios between input and output loads.
2. Related Technical Art
In many applications, it is necessary to transfer electronic signals between components, devices, or systems having different input/output load impedances. That is, the characteristic output impedance for one device or system may differ from the characteristic input impedance of a corresponding load device connected to that output. Any impedance mismatch results in signal reflection or attenuation and a signal being transferred between the devices or systems may experience severe degradation or noise.
To avoid the deleterious effects of impedance mismatching, a variety of devices or circuits have been used to perform impedance transformations. An impedance transformation device receives signals at an input having a first operating impedance, which approximates the characteristic impedance of an input load, and transfers the signal to an output operating at a second impedance which approximates that of a proposed load. One such device is an impedance matching transformer having windings whose characteristic inductive impedance matches that of the respective input and output loads. In microwave applications, quarter wavelength transformers have proven useful for impedance matching.
However, the use of transformer-type impedance transformation devices is generally limited to fixed frequency or resistive impedance load applications. The inductive nature of the transformer makes its output very dependent on operating frequency and it cannot be designed to accommodate large bandwidths. In addition, since a transformer design is static, a transformer cannot adjust to frequency dependent or adjustable impedance changes in reactive loads.
Another impedance transformation or matching technique is the application of reactively tuned amplifiers. Here, appropriately biased transistors are used to transfer an input signal (power) from a source having a first characteristic impedance to a load with a different impedance over a specified frequency range. Within any amplifier design, transistor or transistor stage impedance matching then becomes an important design problem. There is a natural attenuation or gain roll-off that occurs across the frequency limits. To compensate for transistor impedance matching and roll-off, matched amplifier stages are required. While this is conventionally accomplished, it requires complex circuit designs, careful component selection and matching, and multi-sectional filters among other design considerations. The resulting designs can cover moderate band widths with high power. Unfortunately, this approach has proven to be too complex, costly, and difficult for alignment and tuning for most very high frequency applications. This technique does not lend itself to monolithic, mass produced circuits. In addition, the operating bandwidth of reactive circuits is limited by the reactive matching network which is known to have about a two octave limit.
One proposed method of transferring signals over a very high bandwidth, especially at high frequencies, is to use distributed amplifiers. Such amplifiers achieve very broad band signal amplification and show potential for obtaining up to five to six octave bandwidths. However, distributed amplifiers transfer signals between input and impedances of the same value. If there is a difference in the impedances of the loads, signal degradation occurs and the amplification or gain of the circuit is relied upon to compensate for losses. However, impedance mismatch significantly affects output power and amplifier efficiency, and contributes to high signal reflection which in turn causes poor VSWR.
What is needed in the art is a method or apparatus for transferring signals between elements or loads having widely different characteristic impedances. The method needs to have a very broad bandwidth response at very high frequencies. It would be advantageous to obtain appreciable signal gain during transfer with very flat response over the apparatus bandwidth.